That whole passage of scripture from Matthew 11:7 to Matthew 11:15 is about using John's noteriety to sell the appeal of being in the kingdom of the heavens.
In verse 11, Jesus is essentially saying to his audience: "But as impressive as John is, keep in mind that a less impressive one in heaven is still greater than John is."
Upon hearing this some of Jesus' disciples might be saying to themselves: "Well if John is the greatest of those born of women and he's still inferior to a lesser one in heaven, then who am I to be exerting myself to enter this kingdom? Who could possibly qualify? I don't stand a chance!"
Jesus tries to thwart such likely negative reasoning with his statement in verse 12:
"But from the days of John the Baptist until now the kingdom of the heavens is the goal toward which men press, and those pressing forward are seizing it." In other words: "despite your thinking that's its impossible to qualify many men have being seeking it and they will succeed. So don't think that its impossible for you to qualify. Just exert yourself"
So Jesus was essentially using the fame of John to impress upon his audience that being in the kingdom of the heavens is something that they should crave even more than John's fame and spiritual standing. It's as if he's saying: 'You think John is impressive, do you? Well then reach out for the kingdom of the heavens because the glory and favor of a lesser one in such a position is even greater than theglory and favor that John currently has. So reach out for it and you can have it!"
Watchtower has overthought the verse to make it seem like Jesus was saying John wouldn't be in heaven. But that's not the point Jesus was making. If John would go to heaven he would then be greater than he was when on earth. So the comparison can't be seen as an indication or revelation that John wouldn't go to heaven. Jesus was comparing the standing of someone already in heaven with John's then impressive standing on earth. To illustrate the folly of Watchtower's reasoning. Imagine if Jehovah had told Samuel that the glory that the glory and raiment of a king is greater than that of the shepherdboy David. Would that have meant that David would not be appointed as king? No.